1. Field of the Invention
This invention relates to gas turbines in which cooling air is introduced into the interstage disc cavities containing the stator to rotor shaft seals. More particularly, it relates to an arrangement which confines the ingress of hot main gas flow into the interstage disc cavities to regions capable of withstanding high temperatures, thereby reducing the cooling air requirements to provide increased turbine efficiency.
2. Background Information
Gas turbines such as those used to drive electric power generators have a number of rotor discs axially spaced along a rotor shaft to form interstage disc cavities. Stages of the stator extend radially inward from the turbine casing into the interstage disc cavities. Each stator stage includes a number of stator vanes secured to the turbine casing and a seal assembly which seals against the rotor shaft to prevent main gas flow from bypassing the vanes.
The stator sections of the turbine form with the upstream rotor discs annular subcavities within the interstage disc cavities. Cooling air bled from the turbine compressor is introduced from the stator shaft into the interstage disc cavities to cool and seal the seal assemblies. The cooling air flows radially through the interstage disc cavities, including the subcavities, and passes outward through a rim seal into the main gas flow.
Despite the provision of the rim seal and an adjoining rim seal cavity at the exit of the subcavity, some main gas flow ingresses into the subcavities. Pressure variations induced by the rotating parts cause recirculation within the subcavities, thus drawing the very hot main gas flow toward the stator to rotor seals. Sufficient cooling gas must be provided to protect these seals from the hot main gas ingress. This reduces the overall efficiency of the gas turbine.
There is a need, therefore, for an improved gas turbine with increased efficiency.
More particularly, there is a need for a reduction in the volume of cooling air needed to cool components in the interstage disc cavities of a gas turbine.
There is a more specific need for an arrangement which reduces heating within the interstage disc cavities of a gas turbine due to ingress of main gas flow into the interstage disc cavities.
These needs, and others, are satisfied by the invention which is directed to an improved gas turbine which reduces the volume of cooling air needed for cooling the interstage disc cavities by confining the ingress of hot main gas flow to regions of the interstage disc cavities which can withstand high temperatures. More particularly, the invention is directed to a gas turbine comprising a turbine casing and a rotor mounted for rotation within the casing and comprising a rotor shaft with at least first stage and second stage rotor discs axially displaced on the rotor shaft to form an interstage disc cavity. A stator has at least one stator stage extending radially inward into the interstage disc cavity from the turbine casing toward the rotor shaft. The stator stage has a plurality of stator vanes axially aligned with rotor blades carried by the rotor discs and terminates radially inward with a seal assembly which seals against the rotor shaft. The stator stage forms with the first stage rotor disc an annular subcavity within the interstage disc cavity. A cooling system within the rotor shaft introduces into the interstage disc cavity cooling air which passes radially outward through the interstage disc cavity including the subcavity and is discharged into the main gas flow. The gas turbine of the invention also includes a baffle extending from the seal assembly partially across the subcavity toward the first stage rotor disc. The baffle divides the subcavity into a radially inward region and a radially outward region. The baffle is configured and positioned to confine ingress from the main gas flow into the radially outward region. Thus, the radially inward region is protected from the hot main gases. This permits the volume of the cooling gas to be reduced, resulting in an increase in efficiency of the turbine.
The baffle is an annular flange secured to the seal assembly. Where the stator stage includes bolts connecting the seal assembly to the stator vanes, and these bolts have heads projecting axially into the subcavity, the baffle is positioned radially outward of the bolt heads, so that they are in the radially inward region of the subcavity and protected from the ingress from the main gas flow. Again, the baffle is preferably an annular flange and extends axially from the seal assembly beyond the bolt heads. The baffle extends axially at least ⅓ and not more than ⅔ across the subcavity and preferably from between about xc2xd and ⅔. In the most preferred arrangement, the baffle extends about ⅔ across the subcavity.
Similar baffles can be provided in the additional downstream subcavities within an additional interstage disc cavities in the gas turbine.